Lightweight structures particularly as used in the aircraft technology and in spacecraft technology, frequently comprise an outer skin, the inwardly facing surface of which is reinforced by a frame structure which herein is referred to as a “two-dimensional” stiffening, compared to the entire body which is “three-dimensional”. The aircraft body is constructed of longitudinally extending stringers and circumferentially extending ribs to which the outer skin is secured, whereby the skin is reinforced by the stringers and ribs. In designing such lightweight structures special attention is paid to reducing weight. Further, lightweight structures that are used for different purposes will have different strength requirements and may need to satisfy different fatigue characteristics as well as different tolerances with regard to damages to such structures. Lightweight structures particularly used in aircraft construction must additionally satisfy special regulation requirements with regard to the tolerance characteristics that must be satisfied relative to damages that can occur during use of the aircraft.
Increasing the tolerance against damages or damage tolerance of such lightweight structures can be accomplished in different ways, for example, among other things, by increasing the entire skin thickness, or by providing different skin thicknesses in different locations throughout the lightweight structure so that the skin is thicker in locations exposed to higher loads while the skin is thinner in locations exposed to lesser loads. Strengthening the skin by increasing the thickness of the skin even only locally, increases the weight more than can be tolerated. Another possibility of increasing the skin strength resides in using materials which themselves have improved tolerances against damages. Such materials are disclosed in German Patent Publication DE 102 38 460 A1, which describes metallic laminated materials or fiber composite laminates as are on the market under the Trademark GLARE®.
Fiber reinforced laminated materials have the advantage of a very good tolerance against damages, even though these fiber composite materials have a relatively low density compared to monolithic metallic materials. The term “monolithic” as used herein refers to single layer materials primarily of metals, as opposed to multi-layer laminated materials. Conventional fiber composite materials have, to some extent, static strength characteristics that are not as good as such static strength characteristics of monolithic materials. Due to the lower static strength characteristics of fiber composite materials a weight reduction of the entire lightweight structure is possible only in certain areas which primarily are designed with due regard to the good damage tolerance of these materials. Furthermore, the production of fiber reinforced laminated materials is subject to a substantial effort and expense compared to monolithic sheet metals, due to the needed preparation of thin sheet metal layers for the adhesive bonding with additional prepreg films and due to the necessity of manually positioning and preparing for the following adhesive bonding step. As a result, the production costs for laminated composite materials can be significantly higher than the costs for producing monolithic sheet metals. Noticeably smaller costs are involved in the production of metallic laminated materials without a fiber reinforcement as described in the above mentioned German Patent Publication DE 102 38 460 A1.